In the case of conventional forced or induced draft burners employed within heater unit categories such as steam generators, boilers, tube heaters, process furnaces and process heaters, motor-driven fans or blowers provide the means of creating a forced or induced draft of atmospheric air into a heater unit's burner unit assembly.
For complete combustion of varied gaseous or liquid fuels, a 120% to 200% quantity of stoichiometric air was typically employed in earlier conventional heater and burner designs. The long resident times, in which high temperature extended length combustion was maintained within the earlier heaters, resulted in extremely high flue gas emissions. The supply of air is increased above stiochiometric level to both supply an excess of oxygen (hereafter may be referred to as O.sub.2) for completed combustion and for added fluid mass flow heat absorption to lower example air/methane stoichiometric adiabatic combustion flame zone gas temperatures below approximately 3300° F. Molecular portions of the predominant nitrogen (hereafter may be referred to as N.sub.2) mass within the air that are subsequently raised to temperatures 2600° F. and above, are susceptible to entering into completed endothermic chemical reactions that converts nitrogen to nitrogen oxide (hereafter may be referred to as NO. sub.x). Within the typical two or more air-fuel combustion zones, portions of the combustion zones produced carbon dioxide (hereafter may be referred to as CO.sub.2), and any CO.sub.2 that can be are introduced as a molecular component of re-circulated flue gas, can be subsequently raised to temperatures greater than 2600° F. and thereby being susceptible to entering into completed endothermic chemical reactions that converts CO.sub.2 into carbon monoxide (hereafter may be referred to as CO).
Conventional air-gas fuel fired burners are typically of a premix design, wherein a portion of the heater's air for burning is premixed with gaseous fuel before the fuel reaches a point at which combustion is initiated. A conventional partial-premix burner's function is to maximize the homogeneous mixing of specific amounts of air and fuel and cause the fuel to burn completely through exothermic chemical combustion processes within two or more zones of staged combustion. Typical current art low-NO.sub.x heater unit burners can employ a lean-burn first stage of combustion wherein a large excess of stoichiometric air is mixed with the first stage introduced fuel to insure complete fuel combustion, subsequently followed by second stage combustion wherein sufficient additional fuel is introduced into the rich oxygen bearing combustion gas stream emanating from the first stage of combustion to achieve an example excess 6% oxygen content in the second stage combustion exhaust.
Early styles of conventional air-liquid fuel burners are generally of the non-premix type, unless the liquid fuel can be converted into a vapor through atomization and vaporization immediately prior to burning.
Current art air-fuel heater Low-NO.sub.x burners can be of greatly varying designs that incorporate individual manufacturer's various preferred configurations of two or more combustion zones and flame patterns of varied shape, volume and length. The majority of current art heater applications employ burner assemblies whose actual zones of combustion reactions are projected into and carried forth within the boiler, steam generator, or process heater having an internal plenum or firebox space surrounded by heat transfer coils or tubes contained within. In the case of tube type heater applications, the burner assemblies actual zones of combustion reactions are projected into and carried forth internally within the fluid immersed heat exchanger tubes.
Within the last two decades, applications of industrial furnaces producing charges of molten materials and requiring higher product quality, lower operating costs, and lower fugitive exhaust emissions have collectively motivated the development of many varied and patented designs of oxy-fuel burners. These applications typically require furnace temperatures in the range of 3000° F. to 5000° F. To supply these temperature requirements, oxy-gas fuel burner designs provide the ignition, fuel and oxidant mixing means to develop the required projected flame temperature into the furnaces. The combustion flame can comprise one or two chemical reaction zones and the oxidant can be pure oxygen or a mixture of high purity oxygen and air. Re-circulation of interior furnace combustion gas can be aspirated into the burner by employed typical 10 bar high-pressure oxygen's expansion and developed high velocity within the burner, thereby reducing the combustion flame temperature from the potential maximum of 5000° F. Employment of an oxygen/air mixture in the oxy-fuel burner, to achieve a reduced combustion flame temperature, will result in the production of NO.sub.x in addition to the already produced carbon monoxide emissions.
The primary decades-old objective of a heater's burner design has been one of a burner design that provides stable and economical fuel burning throughout a normal prescribed range of operating conditions. More recently, this objective has been supplemented with the added objectives of both reducing fugitive emissions comprising NO.sub.x, CO, unburned hydrocarbons, and the reduction of CO.sub.2 within emitted flue exhaust gases. To achieve these new added objectives, a means of achieving consistent low controlled combustion zone temperatures and completed exothermic chemical combustion reactions are required by a heater's burner assembly. Together with the noted fugitive emission reduction requirements, a unit heater of higher thermal efficiency will yield a lower consumption of hydrocarbon fuels that result in lower CO.sub.2 emissions for a given heater's net fuel energy conversion requirement.
It has been well known and practiced for many decades that in the presence of higher humidity air or injection of water or steam into a conventional burner air supply stream or fuel combustor zone, there is an increase in combustion flame speeds and higher fuel combustion thermal efficiencies. With the development of exhaust emission measurement devices, it was later determined that the injection of water or steam also produced a very significant 40% to 60% reduction in existing 300 to 400 ppmv NO.sub.x and CO emissions. This reduction in fugitive emissions is achieved however with an accompanying significant added operating expensive for the production of de-mineralized boiler grade water required for steam or liquid water injection to prevent fouling of heat exchanger coils within a given type of heater. Current art Low-NO.sub.x boilers with 83% thermal efficiency and 9 to 10 ppmv NO.sub.x emissions extract and inject produced boiler steam into 2 or more stages of burner combustion.
It has also been well known and practiced for decades that partially re-circulating combustion flue gases containing binary molecular carbon dioxide and water vapor, into a burner's combustion zone, reduces flame temperatures and results in a reduced level of NO.sub.x and CO within a heater's exhaust flue gases. Due to the high temperatures and varied conventional burner designs' flame speeds and non-uniform temperatures throughout all portions of the multi-staged combustion zones, it is not possible to accurately predict what series of altered chemical reactions occur when water vapor and/or carbon dioxide are introduced into a given burner's currently developed individual fuel combustion zones.
To achieve higher heater unit thermal efficiencies, some current art heater designs incorporate a degree of heater exhaust gas circulation and air intake preheating through an air to heater exhaust heat exchanger that is air plenum-positioned between the heater's employed atmospheric air blower or fan discharge and the plenum connected heater's burner assembly. These exhaust heat to air recuperation exchanger designs, when coupled with current technology Low-NO.sub.x premix burners, have been successful in increasing a heater's thermal efficiency upward to an approximate 84% to 86% maximum efficiency value. However, preheating air tends to have the effect of increasing a burner's primary combustion zone temperature thereby increasing the burner susceptibility to increased NO.sub.x generation.
Present employed Low-NO.sub.x burner technology combining exhaust flue gas re-circulation into the heater's fuel burner combustion zone and injection of steam or water frequently produces approximately 25–28 ppmv NO.sub.x or 0.03 lb NO.sub.x within flue gases per million Btu of heater rating. The mass flow rate of employed flue gas re-circulation into the burner's combustion zone has been generally limited to a mass rate range equivalent to 15% to 30% of the ‘produced combustion products mass rate’ for best overall burner performance. This flue gas re-circulation rate equates to approximately 4% to 8% of the total conventional Low-NO.sub.x heater unit's combined exhausted flue gases.
Present best-employed Ultra-Low NO.sub.x burner technology to achieve natural gas fuel combustion exhaust emissions containing less than 9 to 10 ppmv NO.sub.x predominantly involves the large industrial heater manufacturer's utilization of selective catalytic reduction (hereafter may be referred to as SCR) assemblies with limited life cycles and higher operating and installed capital costs. In the SCR system, the exhaust gas first passes through an oxidizing catalyst that oxidizes carbon monoxide and unburned hydrocarbons into carbon dioxide and water vapor. A reagent is then mixed into the exhaust stream before its passage through a second catalyst (usually vanadium pentoxide), which results in the selective reduction of nitrogen oxide to form nitrogen and water vapor. These catalytic assemblies employ urea or ammonia as a required chemical reagent with the catalysis. The infrastructure to support wide scale and efficient distribution of these chemical reagents for nitrogen oxide emission reduction purposes has not been developed to date. SCR systems are generally not applied to liquid fuel burner applications, because the catalysts are fairly intolerant of sulfur or metallic elements prevailing within refined fuel oils.
Present best-employed burner technology to achieve low liquid fuel combustion exhaust emissions is primarily limited to the employment of: high pressure preheated liquid fuel to speed vaporization of the liquid fuel during injection within multiple high efficiency premix sub-assemblies contained within a burner assembly; and the injection of steam or boiler grade water into the burner fuel combustion zone. The rate of boiler grade water or steam injection can range between 0.10 to 0.30 lbs per lb of fuel.
In the case of best present day liquid fuel combustion, the present difficulties of rapidly completing a liquid fuel's complete combustion, while controlling an acceptable low combustion zone gaseous temperatures with current combustion techniques, results in elevated levels of fugitive emissions comprising nitrogen dioxides, carbon monoxide, and unburned hydrocarbons that are unacceptable in many environmental control area jurisdictions.
A present low-NOx heater system employs forced or induced draft supply of 106% stiochiometric atmospheric air as a source of oxygen that acts as an example methane fuel combustion oxidizer reactant and nitrogen comprises approximately 72% of the post-combustion zone gases. Due to its diatonic molecular structure, the nitrogen molecules within combustion zones are capable of absorbing and transferring combustion heat only through continued convective heat transfer means resulting from their collisions with other gaseous molecules of greater or lesser temperature.
The conventional and Low-NO.sub.x burner and burner design's inherit delay in rapidly establishing a sufficiently low combustion zone equilibrium gas temperature uniformly throughout its combustion zones, enables portions of the very high temperature predominant nitrogen molecular mass and lesser carbon dioxide molecular mass to enter into completed endothermic chemical reactions that produce NO.sub.x and CO.
To achieve the objective of greatly reducing a heater unit burner assembly's fugitive exhaust emissions and increasing thermal efficiencies and stable fuel burning throughout a normal prescribed range of operating conditions, it is necessary to alter both the fuel combustion chemical reaction formula and the means by which an acceptable range of controlled combustion zone temperatures can be uniformly maintained and distributed with greatly accelerated combustion heat transfer means.